Millions of people experience difficulties with their vision due to a number of common refractive conditions such as myopia (nearsightedness) and hyperopia, (farsightedness). Myopia is a visual abnormality where an eye cannot focus on far-away objects because the cornea is curved too steeply and/or the eye is elongated axially front to back, such that it fails to provide a sharp focus of light at the retinal plane of the eye. On the other hand a hyperopic eye may focus on either far or near objects because the curvature of the cornea of the eye is too flat and/or too short axially front to back to provide adequate focusing at the retinal plane of the eye.
Another common refractive vision problem is astigmatism, which may exist in combination with either myopia or hyperopia. With astigmatism, unequal curvatures of one or more refractive surfaces of the eye prevent light rays from focusing sharply at the plane on the retina, thereby resulting in blurred vision. Yet another common vision problem is presbyopia, which may occur with other refractive problems such as hyperopia, myopia, and/or astigmatism. Presbyopia is the most common vision problem in adults 40 years of age and older. At this age, many people begin to experience difficulty focusing on close objects, most commonly due to the loss of flexibility of the eye's focusing apparatus.
There exist numerous known methods for treating myopia. One conventional method of correcting the visual blur caused by myopia involves wearing a pair of concave (minus powered) spectacle lenses. In some cases, the visual blur caused by myopia may be corrected with concave (minus powered) contact lenses. In the 1970's and 1980's attempts were made to permanently correct myopia through surgical procedures such as radial keratometry (RK). More recently, approaches to correcting myopia through laser surgical reshaping of the cornea (e.g., PRK, LASIK, LASEK) have become popular.
Orthokeratology is a further approach for treating myopia, wherein the corneal shape is altered by wearing rigid contact lenses designed to continually exert pressure on selected locations of the cornea to gradually mold the cornea into the desired corneal curvature. Referring to FIG. 1, a cornea 250 is depicted in a manner showing each of the five layers of tissue: epithelium 200, Bowman's layer 210, stroma 220, Descemet's membrane 230 and endothelium 240. With reference now to FIG. 2, a cornea 2 is depicted including an epithelium 3 comprising a thin layer of cells that cover the surface of the cornea. Epithelium includes an outer surface 4 and an inner surface 8. In FIG. 2, the cornea 2 is shown in juxtaposition with a known contact lens 6, wherein a centerline 20 of the cornea 2 is indicated for purposes of orientation. During conventional orthokeratology treatment, the outer surface 4 of epithelium 3 is compressed against the inner surface of a relatively flat contact lens 6 such that it is believed to be thinned, thus creating a refractive adjustment or correction in the cornea. In actuality, FIG. 3 represents the actual tissue shape change.
After the refractive adjustment, a retainer lens is worn on a part-time basis to prevent the cornea from returning to its previous shape. This “maintenance period” lasts for an extended amount of time after removal of the corrective rigid contact lens. The successfulness of orthokeratology treatment depends on various factors including the shape and structure of the contact lens. For example, a conventional contact having a central radius of curvature that is larger than the central radius of the cornea (i.e., a “flat” contact lens) is widely believed to change the shape of the cornea. The reshaped cornea has a lengthened radius of curvature in its central zone, thereby reducing or eliminating the myopia.
Orthokeratology (“OK”) has been performed with varying degrees of success since the early 1970's. Three factors that impact the effectiveness and desirability of orthokeratology procedures and lenses include: (1) the time needed to achieve the desired visual correction; (2) the amount of myopia that can be corrected using orthokeratology; and (3) the maintenance period before the correction degrades. Regarding the time needed to achieve the desired visual correction, conventional orthokeratology techniques typically require an extended amount of time to accomplish a relatively small amount of myopia reduction. In addition, conventional orthokeratology techniques and lenses provide an inadequate maintenance period duration. In particular, a patient must frequently wear a retainer lens in order to increase the time between corrective sessions with the orthokeratology lens.
Corrective lens design has generally relied on an understanding that the OK effect is based primarily on changes in the corneal epithelium. Many theories have been presented to explain the changes in corneal tissue that result in the refractive changes in OK. Changes in corneal tissue that result in the refractive changes in OK have been historically attributed to central epithelial cell compression, thinning and migration and/or mid-peripheral thickening, mid-peripheral hyperplasia, increased cell retention, or decreased epithelial sloughing. However, evidence has revealed that changes in corneal tissue that result in refractive changes in OK is attributed to characteristics of the corneal stroma.
The cornea's stroma is made of a protein called collagen that forms into fibril layers. Mid-peripheral thickening is caused by the lengthening of the collagen fibrils locally, without a chord diameter change, thus shortening its radius of curvature, steepening the corneal surface and increasing the corneal sagitta in the mid-periphery. Central flattening of the corneal surface is caused by the shortening of the collagen fibrils locally, without a chord diameter change, thus lengthening its radius of curvature, and decreasing the corneal sagitta in the central treatment zone. The change of length and radius that occurs in the fibril is caused by specific and measurable pressure gradient changes that may be attributed to contact from a surface of an OK lens.